This invention relates to a method of extracting a value of a spectral envelope parameter which specifies a logarithmic spectrum related to an input signal and which may simply be called an envelope parameter. This invention relates also to a device for use in carrying out the method.
It is important to accurately extract the value of the envelope parameter specifying the logarithmic spectrum of a speech signal in order to reduce the amount of information of the speech signal for storage or for transmission and to carry out speech recognition. It is often necessary to extract the value of the envelope parameter for other signals.
On extracting the value of the envelope parameter for the speech signal, a conventional method is to extract the value of the envelope parameter from a cepstrum related to the speech signal. The cepstrum is obtained by carrying out an inverse Fourier transform of a logarithmic spectrum of the speech signal. As will later be described in detail, the cepstrum is given along an axis of frequency and has a first and a second frequency component which indicate an approximate envelope of the logarithmic spectrum and a fine configuration of the logarithmic spectrum, respectively. From the cepstrum, the first frequency component is extracted to provide the value of the envelope parameter. However, the first frequency component does not indicate a true envelope of the logarithmic spectrum but merely the average of the fine configuration of the logarithmic spectrum. The conventional method is therefore incapable of providing an accurate value of the envelope parameter.
An improved method is disclosed by Satoshi IMAI and Yoshiharu ABE in a Japanese technical paper, "Densi Tusin Gakkai Ronbunshi (A)" (The Transactions of the Institute of Electronics and Communication Engineers of Japan, (A)), Vol. J62-A (1979), pages 217-223 (April), under the title of "Kairyo Kepusutoramu Ho ni yoru Supekutoru Horaku no Chushutsu" (Spectral Envelope Extraction by Improved Cepstral Method). The method of Imai et al provides the accurate value of the envelope parameter by producing a corrected frequency component indicative of the true envelope of the logarithmic spectrum. As will later be described in detail, the corrected frequency component is produced by correcting the first frequency component by the use of the second frequency component. However, the Imai et al method requires a great number of operations in order to produce the corrected frequency component from the first frequency component. This is because correction of the first frequency component needs calculation of the Fourier transform repeatedly several times and of the inverse Fourier transform also several times.